My Son Is 3 And He Throws Up All The

My son is 3 and he throws up all the time we have been to his Dr. repeatedly and they said it is acid reflux and have given him meds for that. I do not belive that is what is going on and need to find out why my child throws up all the time, I had to have my gallbladder out when i was 13 and they said that it had never worked because my mother went through what I am going through with my son with me for 10 years . He will say his stomach hurts and then he does it and he is to young to understand when i ask what kind of pain is it. The vomit is slimey and smells really bad. I have tried to pin point if it is something he eats and really its not because he can have a glass of milk and do it. Do you have any idea what this could be or where i can turn to find some help for him?